HYDRATION O F
[CONTRIBIJTION FROM
THE
1,2-DIMETHYLCYCLOHEXENE
91 1
DEPARTVENT O F CHEMISTRY O F IOWA STATE UNIVERSITY]
The Steric Course of Hydration of 1,2-Dimethylcyclohexene CAROL H. COLLINS'
AND
GEORGE S. HAMMOND2
Received December 14, 1969
Hydration of 1,2-dimethylcyclohexene in aqueous nitric acid gives nearly equal amounts of cis- and trans-l,2-dimethylcyclohexanol. The result is consistent with any mechanism in which the product-determining step involves hydration of a carbonium ion
Much information, both kinetic3-12 and isotopic,13 has been presented on the mechanism of the acid-catalyzed hydration of olefins to tertiary alcohols. On the basis of this evidence, TaftEproposed a mechanism for the hydration which involved both a a-complex and a classical carbonium ion, as shown in equations 1-3. Step 1, the protonation of the double bond to form the a-complex, and step 3, the conversion of the carbonium ion to to the alcohol, are considered to be relatively rapid; while step 2, the interconversion of the acomplex and the classical carbonium ion, is regarded as rate-determining.
the electron cloud,14ie., a nonequivalent proton.13 This is distinguished from both the bridgedhydrogen specieslS and the transition state of Equation 2, which resemble the a-complex in geometry but have more of the electronic character of carbonium ions. l6 Kinetic evidence indicates, but does not compel, the conclusion that the ratedetermining step does not involve a proton transfer13 and that no water is tightly bound to the system in the transition state.l0?l1 Such a mechanism would not be expected to be highly stereospecific if a planar carbonium ion is formed in reaction 2. Even if the lifetime of the cation were short in comparison with rotation about the C,-CB bond, the direction of approach of the \ / molecule should be determined largely by C=C + H30+ Fast -C=C+ HzO (1) water steric interactions with the substituents attached / \ I I n-complex to Cb.Any steric preference introduced by such an effect would probably result in preferential cis H A addition. Recently, work with several strong acids has shown a trans stereospecificity to the addition recarbonium ion action. Thus, Hammond and Nevitt" have observed a clean, stereospecifically trans addition of I ! Fast 1 1 H-C-C--. + 2H20 + H-C-C-OH + H30+ (3) hydrogen bromide to 1,2-dimethylcyclohexene; i f 1 1 Hammond and Collins18 have observed a similar Further refinements by Taft have described the trans stereospecificity with hydrogen chloride and n-complex as nearly olefinic, with a trigonal, 1,2-dimethylcyclopentene; Winstein and Holnesslg coplanar structure and the proton embedded in have observed a stereospecific attack of formic acid on 4-t-butylcyclohexene, and Schleyer20 has ob(1) Department of Chemistry, University of Wisconsin. served a stereospecific proton addition in the re( 2 ) Division of Chemistry, California Institute of Technology, Pasadena, California. Inquiries and requests for re- action of 1-methylnorbornene with formic acid and with ethanolic hydrochloric acid. In each case, prints should be sent to this author. (3) H. J. Lucas and W.F. Eberz, J . Am. Chem. SOC.,56, the stereospecificity observed was explained by 460 (1934). direct attack of the anion on the a-complex or, (4) H. J. Lucas and Y. P. Liu, J . Am. Chem. SOC.,56, a t least, a sufficiently concerted opening of the 2138 (1934). complex, coupled with attack of the anion, to ( 5 ) F. G. Ciauetta and hl. Kilpatrick, J . Am. Chem. Soc., 70; 639 (1948). yield stereochemically pure product. (6) J. B. Levy, R. W. Taft, Jr., D. Aaron, and L. P. The current work on the hydration of 1,2-
r"+
Hammett. J . Am. Chem. SOC..73, 3792 (1951). (7) R. %. Taft, Jr., J. B: Levy, D. Aaron, and L. P. Hammett, J . -4m. Chem. SOC.,74, 4735 (1952). (8) R. W. Taft, Jr., J . Am. Chem. SOC.,75, 1253 (1953). (9) J. B. Levy, R. W. Taft, Jr., and L. P. Hammett, J . Am. Chem. SOC.,75,1253 (1953). (10) J. B. Levy, R. W. Taft, Jr., D. Aaron, and L. P. Hammett, J. Am. Chem. SOC.,75,3955 (1953). (11) E. L. Purlee, R. W. Taft, Jr., and C. A. DeFazio, J . Am. Chem. SOC.77, 837 (1955). (12) R. W. Taft, Jr., E. L. Purlee, P. Reisz, and C. A. DeFazio, J . Am. Chem. SOC.,77, 1584 (1955). (13) E. L. I'urlee and R. W. Taft, Jr., J . Am. Chem. SOC., 78,5807 (1956).
(14) L. G. Cannel1 and R. TV. Taft, Jr., J . Am. Chem. SOC.,78,5812 (1956). (15) M. J. S. Dewar, J . Chem. SOC.,406 (1946). (16) P. Riesz, R. W. Taft, Jr., and R. H. Boyd, J . Am. Chem. SOC.,79,3724 (1957). (17) G. S . Hammond and T. D. Nevitt, J . A m . Chem. SOC., 76,4121 (1954). (18) G. S. Hammond and C. H. Collins, J. Am. Chem. Soc., in press. (19) S. Winstein and N. J. Holness, J . Am. Chem. Soc., 77,5562 (1955). (20) P. von R. Schleyer, J . Am. Chem. SOC.,in press.
912
COLLINS AND HAMMOND
VOL.
25
TABLE I PRODUCT COMPOfiITION I N
-
Concn. Nitric Acid
Run KO. 1 2
3 4 5 a
HYDRATION OF 1,2-DIMETHYLCYCLOHEXENE
Time, Days
0.1dT 0.1M 0.1M 0.1M
7 30
1. O M
5
Temp. 50 50 50 50 28
1 5
Alcohol trans 52 64 58 55 55
cis
48 36 42 45 45
Recovered Alkene,a % ' 1,22,3-
89
11
93
7
Original mixture contained 85y0 1,2-dimethylcyclohexene and 15y0 2,3-dimethylcyclohexene.
dimethylcyclohexene was undertaken to resolve the question of the stereochemistry. RESULTS AND DISCUSSION
Table I lists the relative amounts of the cis- and trans-1,2-dimethylcyclohexanolsformed in the hydration experiments. Although the total conversion to alcohols was small (less than 10% of the total sample analyzed), with virtually all of the rest of the material being unchanged olefin, the percentages observed for the cis- and trans-alcohols indicate nearly equal amounts of each isomer in both 0.1M and 1 . O M nitric acid solutions. The composition of the remaining olefin is also noted. Thus, it is seen that the olefin composition changes little from its original value (%yo 1,2-dimethylcyclohexene) in the more dilute acid, while some isomerization is detectable in the more concentrated acid solution. The actual equilibrium mixture contains even greater amounts of the symmetrical alkene. To determine if equilibration of the cis- and trans1,2-dimethylcyclohexanols were causing the near equivalence of the isomeric composition in the hydration experiments, several different alcohol mixtures were subjected to similar treatment. Results are given in Table I1 and indicate that, within experimental error, no (dilute) acid-catalyzed isomerization was taking place. TABLE I1 EQUILIBRATION STUDIES WITH 1,z-DIMETHYLCYCLOHEXANOLS IN 0.1M NITRICACID
Run Yo. 1
2 3 4
5 6
% Composition Starting Recovered Material Material trans cis trans cis 100 100
43 43 27 2i
0 0 57 57 73 73
100 100 48 42 23 26
0 0
52 58 77 74
Time, Days 1 1 1 1
3 1
As the nearly equivalent amounts of cis- and trans-l,2-dimethylcyclohexanol found in the hydration of l12-dimethylcyclohexane are not the result of an isomerization of a stereospecifically
formed alcohol but rather are the actual results of the hydration itself, the results are in agreement with any mechanism, including that of Taft,s in which the product determining step involves hydration of an open carbonium ion.21 As the stereochemical course of hydration is different from that observed in addition of certain acids in organic solvents, there must be some mechanistic differences. However, the variations suggested are both small and of a character which might be anticipated. Both types of reaction can be reasonably formulated as involving complexes between an alkene and a proton (or molecular acid). If it is granted that such complexes maintain their integrity and react directly with a nucleophile in organic solvents, one could hardly devise conditions more favorable for allowing the complexes to isomerize to a carbonium ion than those obtaining in the hydration studies. The reaction is carried on in a medium (water) of high dielectric constant with a good capacity for specific solvation of cations, and the nucleophile (water) ultimately involved in the reaction is less reactive than those involved in most of the stereo-specific reactions. The fact that two electrophilic addition reactions, hydrogen bromide adition and hydration, with a single substrate, 1,2-dimethylcyclohexene, give different stereochemical results indicates that predictions of the stereochemistry of such reactions will probably be unsafe unless they are based upon very close analogies. Similar pessimism is probably also warranted concerning other problems, such as prediction of the likelihood of molecular rearrangements in the course of addition reactions. EXPERIMENTAL
Materials. cis- and trans-l,&Dimethylcyclohexano1 were prepared by the method of Nevitt and Hammond.22 Physical constants were: cis-alcohol, b.p. 78.9-79.6' a t 25 mm. (21) A referee has suggested that, if the rate of hydration of 2,3-dimethylcyclohexene is much faster than that of the 1,2-isomer, most of the alcohols could come from that compound and mixtures could be produced by concerted, transaddition. We agree, but feel that this is unlikely in view of the usud reactivity relationships among variously subst,ituted alkenes. (22) T. D. Nevitt and G. S. Hammond, J . Am. Chem. floc., 76,4124 (1954).
JUNE
1960
ADDITIONS TO CYANOSTYRENES
(lit.,22 b.p. 95.7 at 53 mm.) and trans-alcohol, b.p. 72.472.9' at 25 mm. (lit.,89 b.p. 86.8" at 52 mm.). 1,d-Dimethylcyclohexene was prepared by dehydration of the mixed alcohols by heating the mixture with iodine. The distilled olefin, b.p. 136.0-136.1' at 735 mm. (lit.,lT b.p. 135.4-135.9") was shown to contain 85% 1,a-dimethylcyclohexene and 15% 2,3-dimethylcyclohexene by gas chromatographic analysis on a 6-foot column using Apiezon L grease as the liquid phase. The olefin peaks were distinct and well separated. Calculation was by triangulation,z3 and the results are reproducible to &2%. The infrared spectrum of this sample was identical with that obtained by the previous workers." Procedure. For the hydration experiments, 15 ml. of aqueous nitric acid (1.0 or 0.1M) was placed in a small flask and 1 g. of 1,2-dimethylcyclohexene added. The flask was fitted with a condenser and magnetic stirring bar and placed in a constant temperature bath (28' or 50") where the solution was stirred for periods varying from 1 to 30 days. At the end of these intervals, the sample was extracted with pentane. (23) A. I. M. Keulemans, Gas Chromatography, Reinhold Publishing Corp., New York, N. Y. (1957), p. 32.
'313
The pentane solution was washed with water and sodium bicarbonate solution and dried briefly over anhydrous calcium sulfate. The pentane was then removed by distillation, and the products were analyzed by gas chromatography a t 150' on a 6-foot column with Apiezon L grease as the liquid phase. The cis- and trans-alcohol peaks showed some overlapping but calculation (by triangulation)22 of the amounts present was poseible. However, because of this overlapping, the results are probably accurate only to 13-4%. For the equilibration experiments, 1 g. of an alcohol mixture of known composition was placed in 15 ml. of 0.lM nitric acid solution. Further treatment was identical with that of the hydration experiments.
Acknowledgment. We gratefully acknowledge support of this reasearch by the United States Army Office of Basic Ordinance Research. We are also indebted to Dr. Paul Schleyer for prepublication material and to Professor Robert Taft for many enlightening discussions of this subject. AMES,IOWA
[CONTRIBUTION FROM THE DEPARTMENT O F CHEMISTRY, MONTANA STATE UNIVERSITY]
Addition of Nucleophilic Reagents to 0- and p-Cyanostyrene JOHN M. STEWART, IRWIN KLUNDT,
AND
KENNETH PEACOCK
Received November 6, 1969 A new improved synthesis of o-cyanostyrene was developed. Reactions of a variety of amines and thiols with both 0- and p cyanostyrene were carried out and the yields were compared to those of similar reactions previously performed with acyanostyrenes and 2,4-pentadienenitrile.a Results indicate a considerable diminishing of the electron-withdrawing effect of the cyano group when passed through the benzene ring conjugation.
o-Cyanostyrene and p-cyanostyrene may be considered to be vinylogs of acrylonitrile. Vinylogs are related compounds of the type formula A(CH= CH),B, in which the groups A and B are linked through one or more conjugated vinylene groups. An empirical rule developed by Angeli' has been useful in correlating the relations observed in reactions of certain vinylogs. It states that substituent groups situated ortho or para to each other on a benzene nucleus behave qualitatively as though they were joined directly. Accordingly, it would be predicted that o-cyanostyrene might correspond closely to the open chain vinylog, 2, 4-pentadienenitrile (CHF==CH-CH=CH-CN), of acrylonitrile-one conjugated vinylene group being part of the benzene ring. Compounds which contain an alkene linkage directly connected to a highly electron-withdrawing group, or conjugated with it through other vinylene linkages, react by addition across the alkene linkage with nucleophilic reagents which contain labile hydrogen atoms-for example, with primary and secondary amines, thiols, and phenols. (1) A. Angeli, Atti. accad. Link [VI 32i, 443 (1923), C h m . Abstr., 18, 1118 (1924); R. C. Fuson, Chem. Revs. 16, 1 (1933).
Thus, reactions of this type have been carried out with 2,4-~entadienenitrile~ and atroponitrile* (a-cyanostyrene), and the results have been compared with the familiar cyanoethylation reactions of acrylonitrile. It was of interest, therefore, to study the addition of nucleophilic reagents to o-cyanostyrene and p-cyanostyrene and to determine whether the electron-withdrawing effect of the cyano group upon the alkene linkage is diminished when passed through the benzene ring conjugation. o-Cyanostyrene has been prepared by Marvel and Hein4 by the decarboxylation of o-cyanocinnamic acid following a sequence of reactions starting with o-tolunitrile. The compound has also been reported as prepared in a British patent to the Wingfoot Corporation6 by direct chlorination of oethylbenzonitrile, followed by pyrolysis of the chloroethyl derivative to give o-cyanostyrene. It was impossible to duplicate the latter procedure in this investigation. The over-all yield in method (2) J. M. Stewart, J. Am. Chem. SOC.76, 3228 (1954). (3) J. M. Stewart and C. H. Chang, J. Org. C h a . 21, 635 (1956). (4) C . S.Marvel and D. W. Hein, J. Am. Chem. Soe. 70, 1895 (1948). (5) Wingfoot Corporation, Brit. Pat. 571, 829 C h a . Abstr. 41, 3322 (1947).
